5 research outputs found
Development of combustion models for RANS and LES applications in SI engines
Prediction of flow and combustion in IC engines remains a challenging task. Traditional
Reynolds Averaged Navier Stokes (RANS) methods and emerging Large Eddy Simulation
(LES) techniques are being used as reliable mathematical tools for such predictions. However,
RANS models have to be further refined to make them more predictive by eliminating or
reducing the requirement for application based fine tuning. LES holds a great potential for
more accurate predictions in engine related unsteady combustion and associated cycle-tocycle
variations. Accordingly, in the present work, new advanced CFD based flow models
were developed and validated for RANS and LES modelling of turbulent premixed
combustion in SI engines.
In the research undertaken for RANS modelling, theoretical and experimental based
modifications have been investigated, such that the Bray-Moss-Libby (BML) model can be
applied to wall-bounded combustion modelling, eliminating its inherent wall flame
acceleration problem. Estimation of integral length scale of turbulence has been made
dynamic providing allowances for spatial inhomogeneity of turbulence. A new dynamic
formulation has been proposed to evaluate the mean flame wrinkling scale based on the
Kolmogorov Pertovsky Piskunow (KPP) analysis and fractal geometry. In addition, a
novel empirical correlation to quantify the quenching rates in the influenced zone of the
quenching region near solid boundaries has been derived based on experimentally estimated
flame image data. Moreover, to model the spark ignition and early stage of flame kernel
formation, an improved version of the Discrete Particle Ignition Kernel (DPIK) model was
developed, accounting for local bulk flow convection effects. These models were first verified
against published benchmark test cases. Subsequently, full cycle combustion in a Ricardo E6
engine for different operating conditions was simulated. An experimental programme was
conducted to obtain engine data and operating conditions of the Ricardo E6 engine and the
formulated model was validated using the obtained experimental data. Results show that, the
present improvements have been successful in eliminating the wall flame acceleration
problem, while accurately predicting the in-cylinder pressure rise and flame propagation
characteristics throughout the combustion period.
In the LES work carried out in this research, the KIVA-4 RANS code was modified to
incorporate the LES capability. Various turbulence models were implemented and validated in engine applications. The flame surface density approach was implemented to model the
combustion process. A new ignition and flame kernel formation model was also developed to
simulate the early stage of flame propagation in the context of LES. A dynamic procedure
was formulated, where all model coefficients were locally evaluated using the resolved and
test filtered flow properties during the fully turbulent phase of combustion. A test filtering
technique was adopted to use in wall bounded systems. The developed methodology was then
applied to simulate the combustion and associated unsteady effects in Ricardo E6 spark
ignition engine at different operating conditions. Results show that, present LES model has
been able to resolve the evolution of a large number of in-cylinder flow structures, which are
more influential for engine performance. Predicted heat release rates, flame propagation
characteristics, in-cylinder pressure rise and their cyclic variations are also in good agreement
with measurements
Simulation of engine combustion with ethanol as a renewable fuel
Ethanol as a fuel is an important bio-fuel for future energy needs. In this work the combustion process of gasoline-ethanol blends in spark ignition engines was investigated using computational fluid dynamics and turbulent combustion modeling. A modified flame surface density approach developed for gasoline engine combustion was adapted to calculate fuel-burning rate of the blend. The rise in in-cylinder peak pressure and temperature for blends up to E20 was found relatively small compared to E00. A significant reduction of CO and an increment of NOx were observed for optimized combustion with adjusted ignition timing
Simulation of premixed combustion and near wall flame quenching in spark ignition engines with an improved formulation of the Bray-Moss-Libby model
Theoretical and experimental based modifications have been investigated, such that the BML model can be applied to wall-bounded combustion modelling eliminating the wall flame acceleration problem. Estimation of integral length scale of turbulence has been made dynamic so that allowance for spatial inhomogeneity of turbulence is made. A new dynamic formulation has been proposed based on the Kolmogorov- Petrovski-Piskunov analysis and fractal geometry to evaluate the mean flame wrinkling scale. In addition, a novel empirical correlation to quantify the quenching rates in the influenced zone of the quenching region near solid boundaries has been derived based on experimentally estimated flame image data.
The proposed model was then applied to simulate the premixed combustion in spark ignition engines. Full cycle combustion in a Ricardo E6 engine for different operating conditions was simulated. Results show that the present improvements have been successful in eliminating the wall flame acceleration problem, while accurately predicting the in-cylinder pressure rise
An improved formulation of the Bray-Moss-Libby (BML) model for SI engine combustion modelling
In this paper an improved version of the BML model has been developed so that it could be applied to wall-bounded combustion modelling, eliminating the wall flame acceleration problem. Based on the Kolmogorov-Petrovski-Piskunov (KPP) analysis and fractal theory, a new dynamic formulation has been proposed to evaluate the mean flame wrinkling scale making necessary allowance for spatial inhomogeneity of turbulence. A novel empirical correlation has been derived based on experimentally estimated flame image data to quantify the quenching rates near solid boundaries. The proposed modifications were then applied to simulate premixed combustion in two spark ignition engines with different operating conditions. Results show that the present improvements have been successful in eliminating the wall flame acceleration problem found with the original BML model, while accurately predicting the in-cylinder pressure rise, mass burn rates and heat release rates
Large eddy simulation of premixed combustion in spark ignited engines using a dynamic flame surface density model
In this work, cyclic combustion simulations of a spark ignition engine were performed using the Large Eddy Simulation techniques. The KIVA-4 RANS code was modified to incorporate the LES capability. The flame surface density approach was implemented to model the combustion process. Ignition and flame kernel models were also developed to simulate the early stage of flame propagation. A dynamic procedure was formulated where all model coefficients were locally evaluated using the resolved and test filtered flow properties during the fully developed phase of combustion. A test filtering technique was adopted to use in wall bounded systems. The developed methodology was then applied to simulate the combustion and associated unsteady effects in a spark ignition engine. The implementation was validated using the experimental data taken from the same engine.
Results show that, even with relatively coarser meshes used in this work, present LES implementation has been able to resolve the evolution of a large number of in-cylinder flow structures, which are more influential for engine performance. Predicted combustion rate and pressure rise is also in good agreement with the measurements. The limits of cyclic variations are well within the experimentally observed range. It has also been able to demonstrate the limits of cyclic fluctuations to a reasonable degree even with a fewer number of simulation cycles. A significant variation of flame propagation has also been predicted by the simulations